THERMAL CRACKING OF A HYDROCARBON FEED

Abstract: Thermal cracking of a hydrocarbon feed comprising heating the feed in a furnace (1); supplying the feed to a reaction chamber (15); separating the stream from the reaction chamber (15) into a light products stream and into a heavy products stream; and supplying the heavy products stream to a vacuum distillation column (30) to separate the heavy products stream into fractions, wherein supplying the heavy products stream to the vacuum distillation column comprises passing the heavy product stream through a flow control valve (36), passing the heavy products stream upwards through a stand-pipe (38) having such a length that the fluid pressure at the end of the stand-pipe (38) is such that vaporization at its bottom end is suppressed, and subsequently passing the heavy products stream through a transfer conduit (40) into the vacuum distillation column (30), the transfer conduit (40) having such a configuration that the fluid pressure at its outlet matches the fluid pressure in the vacuum distillation column (30).

Description: THERMAL CRACKING OF A HYDROCARBON FEED The present invention relates to thermal cracking of a hydrocarbon feed as described in The Petroleum Handbook, 6th Edition, Elsevier, 1983, pages 279-281. Thermal cracking of a hydrocarbon feed comprises heating the feed at a pressure in the range of from 0.2 to 5 MPa and at a temperature in the range of from 390 to 530 °C; supplying the feed to a reaction chamber; separating the stream from the reaction chamber into a light products stream and into a heavy products stream; and supplying the heavy products stream to a vacuum distillation column to separate the heavy products stream into fractions at a pressure in the range of from 1 to 10 kPa and at a temperature in the range of from 320 to 400 β C. In the known process a conduit is used to pass the heavy products stream to the vacuum distillation column. As the pressure of the heavy products stream is larger than the pressure in the vacuum distillation column, the pressure drop along the conduit should equal the pressure difference between the pressure of the heavy products stream and the pressure in the vacuum distillation column. One way of obtaining this rather high pressure drop along the conduit is to provide the conduit with a restricted opening, for example in the form of a flow control valve. However, when the heavy products stream is throttled over a flow control valve rapid vaporization occurs. This rapid vaporization is accompanied by the formation of very small liquid droplets which cannot easily be handled in the vacuum distillation column. The present invention provides a simple way to suppress vaporization over the restricted opening. To this end the process of thermal cracking of a hydrocarbon feed according to the present invention comprises heating the feed at a pressure in the range of from 0.2 to 5 MPa and at a temperature in the range of from 390 to 530 °C; supplying the feed to a reaction chamber; separating the stream from the reaction chamber into a light products stream and into a heavy products stream; and supplying the heavy products stream to a vacuum distillation column to separate the heavy products stream into fractions at a pressure in the range of from 1 to 10 kPa and at a temperature in the range of from 320 to 400 °C, wherein supplying the heavy products stream to the vacuum distillation column comprises passing the heavy product stream through a restricted opening, passing the heavy products stream upwards through a stand-pipe having such a length that the fluid pressure at the end of the stand-pipe is such that vaporization at its bottom end is suppressed, and subsequently passing the heavy products stream through a transfer conduit debouching into the vacuum distillation column, the transfer conduit having such a configuration that the fluid pressure at its outlet matches the fluid pressure in the vacuum distillation column. The expression "configuration of the transfer conduit' in the specification and in the claims is used to refer to the features of the transfer conduit that contribute to the pressure drop along the transfer conduit, such as the dimensions of the transfer conduit and other flow resistances such as bends or U-turns in the transfer conduit. The invention will now be described by way of example in more detail with reference to the accompanying drawings, wherein Figure 1 shows schematically a line-up for the process according to the present invention; and Figure 2 shows schematically an alternative line-up for the process according to the present invention. Reference is made to Figure 1. The line-up for the process according to the present invention includes a furnace 1 provided with a burner 2 and a stack 3 and a heater tube 4 arranged in the furnace 1. The heater tube 4 is connected to an inlet conduit 7 and an outlet conduit 9. The outlet conduit 9 is connected to the inlet of a reaction chamber 15. The reaction chamber is more clearly described in European patent No. 7 656. The outlet of the reaction chamber 15 is connected by conduit 17 to a separation device in the form of cyclonic separator 20. The cyclonic separator 20 has two outlets, an outlet 21 for light products and an outlet 24 for heavy products. The outlet 21 for light products is connected by a conduit (not shown) to a device (not shown) for further treating the light products. The outlet 24 for heavy products is connected by means of a conduit system 26 to a vacuum distillation column 30. The vacuum distillation column 30 has an outlet conduit 31 which is connected to a vacuum pump (not shown) , a residue outlet conduit 32 for heavy products and an outlet conduit 33 for an intermediate product fraction. Devices for providing a suitable reflux stream to the top part of the vacuum distillation column 30 and stream of stripping medium to the bottom part of the vacuum distillation column 30 are not shown. The conduit system 26 includes a conduit section 35 in which a restricted opening in the form of flow control valve 36 is arranged, a stand-pipe 38 joined to the conduit section 35 and a transfer conduit 40 debouching into the vacuum distillation column 30. The length of the stand-pipe 38 is so determined that, during normal operation, the fluid pressure at the bottom end of the stand-pipe 38 is such that vaporization of the fluid at its bottom end is suppressed. The expression "suppressing the vaporization of the fluid" is used in the specification and in the claims to indicate that at most a small amount of liquid is vaporized (less than about 5% by volume) . The configuration of the transfer conduit 40 is so determined that, during normal operation, the fluid pressure in the transfer conduit 40 at its outlet end 41 matches the fluid pressure in the vacuum distillation column 30. This implies that the transfer conduit 40 is so designed that the friction experienced by the fluid flowing through the transfer conduit 40 equals the difference in pressure at the outlet of the stand-pipe 38 and in the vacuum distillation column 30. In this case the transfer conduit 40 includes several straight lines 43 joined by means of curved connectors in the form of U-shaped connectors 44. During normal operation, 3 000 ton/day of a hydrocarbon feed is continuously supplied to the furnace 1 at a pressure of 3 MPa, the feed is heated at a temperature of 450 β C in the furnace 1. Then heated feed is supplied to the reaction chamber 15 where the feed is allowed to crack. The stream of products from the reaction chamber 15 is separated in cyclonic separator 20 into 900 ton/day of light products removed through outlet 21 and into 2 100 ton/day of heavy products removed through outlet 24. The heavy products are supplied through the conduit system 26 to the vacuum distillation column 30 which operates at a pressure of 5 kPa and at a temperature of 380 β C. In the vacuum distillation column 30 the heavy products stream is separated into 200 ton/day of a gaseous fraction removed through outlet conduit 31, 400 ton/day of intermediate fraction removed through outlet conduit 33, and 1 500 ton/day of a residue removed through residue outlet conduit 32. The pressure drop over the flow control valve 36 is 150 kPa. The length of the stand-pipe 38 is 15 m, the presence of liquid in the stand-pipe 38 prevents vaporization of liquid at the downstream end of the flow control valve 36. The length of the transfer conduit 40 is 70 m, the average internal diameter of the transfer conduit 40 is 50 cm and the transfer conduit includes four U-shaped connectors 44. The fluid pressure at the inlet of the transfer conduit 40 is 60 kPa and the fluid pressure at the outlet of it matches the fluid pressure in the vacuum distillation column 30. Omitting the stand-pipe 38 would result in vaporization at or near the flow control valve 36, which vaporization is uncontrolled and is accompanied by the formation of extremely small liquid droplets. Reference is now made to Figure 2 showing an alternative line-up for the process according to the present invention. Parts of the line-up which are similar to the line-up as described with reference to Figure 1 have got the same reference numeral. In the line-up of Figure 2 the stream from the reaction chamber 15 is separated in a distillation column 50 into a light products stream removed through outlet 51 and into a heavy products stream removed through outlet 52. The heavy products stream is then supplied through conduit system 26 to the vacuum distillation column 30. Devices for providing a suitable reflux stream to the top part of the distillation column 50 and stream of stripping medium to the bottom part of the distillation column 50 are not shown. Curved connectors 44 may have a U-shape as shown in Figures 1 and 2, or the curved connectors may have an L-shape. The presence of liquid in the stand-pipe prevents vaporization of liquid at the downstream end of the restricted opening. Omitting the stand-pipe would result in vaporization at or near the restricted opening, which vaporization is uncontrolled and is accompanied by the formation of very small liquid droplets. In the vacuum distillation column these small fine liquid droplets cannot be separated from the gas stream so that the liquid droplets are entrained with the fraction leaving the vacuum distillation column through the outlet conduit for the intermediate fraction or with the gaseous stream leaving the top of the vacuum distillation column through outlet conduit. This entrainment adversely affects the separation efficiency of the vacuum distillation column. The stand-pipe 38 as described with reference to Figures 1 and 2 is a vertical pipe, it will be understood that the stand-pipe can also be a slanted pipe provided that there is, during normal operation, sufficient liquid in the stand-pipe to prevent vaporization.

Claims: C L A I M S 1. Process of thermal cracking of a hydrocarbon feed comprising heating the feed at a pressure in the range of from 0.2 to 5 MPa and at a temperature in the range of from 390 to 530 °C; supplying the feed to a reaction chamber; separating the stream from the reaction chamber into a light products stream and into a heavy products stream; and supplying the heavy products stream to a vacuum distillation column to separate the heavy products stream into fractions at a pressure in the range of from 1 to 10 kPa and at a temperature in the range of from 320 to 400 "C, wherein supplying the heavy products stream to the vacuum distillation column comprises passing the heavy product stream through a restricted opening, passing the heavy products stream upwards through a stand-pipe having such a length that the fluid pressure at the end of the stand-pipe is such that vaporization at its bottom end is suppressed, and subsequently passing the heavy products stream through a transfer conduit debouching into the vacuum distillation column, the transfer conduit having such a configuration that the fluid pressure at its outlet matches the fluid pressure in the vacuum distillation column. 2. Process according to claim 1, wherein the transfer conduit includes a plurality of straight sections joined by curved connectors. 3. Process according to claim 1 or 2, wherein the stream from the reaction chamber is separated in a cyclone separator in a light products stream and a heavy products stream. 4. Process according to claim 1 or 2, wherein the stream from the reaction chamber is separated in a distillation column in a light products stream and a heavy products stream.

Inventors: POLDERMAN HUGO GERARDUS, SCHRIJVERS FRANCISCUS ANTONIUS, VAN HOOFT LEONARDUS MARINUS MA, VAN VOORST WILLEM CORNELIS JAN, POLDERMAN, HUGO, GERARDUS, SCHRIJVERS, FRANCISCUS, ANTONIUS, MARIA, VAN HOOFT, LEONARDUS, MARINUS, MARIA, VAN VOORST, WILLEM, CORNELIS, JAN
Applicants: SHELL CANADA LTD, SHELL INT RESEARCH, SHELL CANADA LIMITED, SHELL INTERNATIONALE RESEARCH MAATSCHAPPIJ B.V.
Application year: 1994
Main classification: C10G9
IPC codes: C10G7, C10G9